The bolts used for the wind turbine for wind power generation need to be of high strength and of sufficient toughness. Hence, as a steel material of the bolt used for the wind turbine, the chromium molybdenum steel that is of high strength and of sufficient toughness is often used. The chromium molybdenum steel is special alloy steel made from iron (Fe), carbon (C), chromium (Cr) and molybdenum (Mo); namely, the chromium molybdenum steel has the material contents, chromium (Cr) and molybdenum (Mo) in the carbon steel (Fe+C) that includes carbon (C) beside the base content iron (Fe); adding the chromium elements to steel improves the hardenability thereof; adding the molybdenum elements to steel further improves the hardenability thereof; moreover, in a tempering process, the chromium molybdenum steel is less prone to being softened and embrittled.
An example of contents as to the chromium molybdenum steel as a material of the bolts used for the wind turbine is now explained with respect to the percentage of contents [in mass %]; the chromium molybdenum steel includes:
carbon (C) content of more than or equal to 0.33% and less than or equal to 0.38%,
silicon (Si) content of more than or equal to 0.15% and less than or equal to 0.35%,
manganese (Mn) content of more than or equal to 0.60% and less than or equal to 0.85%,
chromium (Cr) content of more than or equal to 0.90% and less than or equal to 1.20%,
molybdenum (Mo) content of more than or equal to 0.15% and less than or equal to 0.30% or more than 0.30%,
phosphorus (P) content of less than or equal to 0.03%, and
sulfur (S) content of less than or equal to 0.03%.
The bolt made of the chromium molybdenum steel has sufficient strength and toughness; thus, there is no special problem in using the bolt for the wind turbine installed in the non-cold climate areas.
On the other hand, in a case of a wind turbine installed in the cold climate area where the ambient temperature becomes as low as minus 40° C., it is required that Charpy impact strength of the material of the bolt used for the wind turbine be equal to or more than 27 [J] as an index of the material toughness with respect to the condition of minus 20° C. However, it is known that, out of a plurality of bolts made of the chromium molybdenum steel, some bolts show Charpy impact strength of more than or equal to 27 [J] as the index of the material toughness with respect to the condition of minus 20° C., while some bolts show Charpy impact strength of less than 27 [J] as the index of the material toughness with respect to the condition of minus 20° C.; namely, in a case where a plurality of chromium molybdenum steel bolt is manufactured, Charpy impact strength for some of the bolts does not meet the Charpy impact strength requirement of equal to or more than 27 [J] as an index of the material toughness with respect to the condition of minus 20° C. Hence, the chromium molybdenum steel bolts that do not satisfy the Charpy impact strength requirement cannot be used as the bolts to be used for the wind turbine installed in cold climate areas.
For the reasons as described above, it is sometimes considered to use a steel material other than chromium molybdenum steel for the bolts to be used in the wind turbine installed cold climate areas; thereby, the material as a substitute of chromium molybdenum steel having the property of a high Charpy impact strength at the lower temperature, namely, the property of higher low-temperature toughness.
The patent reference 1 (JP1996-67950) discloses a martensitic stainless steel with excellent strength and toughness; naturally, this martensitic stainless steel can be used as a material of the high low-temperature toughness. The martensitic stainless steel includes:
carbon (C) content of more than or equal to 0.05% and less than or equal to 0.15% [in mass %],
silicon (Si) content of less than or equal to 2% [in mass %],
manganese (Mn) content of less than or equal to 2% [in mass %], and
chromium (Cr) content from 10% to 20% [in mass %].
Further, in the crystal structure matrices of the martensitic stainless steel, the martensitic stainless steel includes fine carbides of particle size smaller than 2 micron-meters, the fine carbide being homogeneously dispersed in the matrices, in a volume percentage of 1 to 30%; thereby, the original austenite grain size of the material is refined into smaller than or equal to 30 micron-meters. In this way, the toughness of the martensitic stainless steel is improved.
However, in order to produce the martensitic stainless steel that is disclosed in the patent reference 1, a plenty amount of chromium has to be added in the crystal structure; thereby, the precipitation and growth of ferrous temper carbide that enhances the strength of the material of the bolt steel cannot be easily expected; thus, it is difficult to increase the strength of the material proposed in the patent reference 1. Hence, it is difficult to use the proposed material as the bolt material in relation to the wind turbine installed in cold climate areas.
Further, if a material to be usable for the bolt of the wind turbine installed in cold climate areas is used together with the chromium molybdenum steel as a material of the bolt for the wind turbine installed in non-cold climate areas, then it becomes necessary to manufacture the two kinds of materials in separated facilities; hence, the initial cost as well as the running expense increases.
Thus, the background described thus far may bring an idea to classify the bolts made of the chromium molybdenum steel into two categories; the first one is the category of the bolts that are usable as the bolt of the wind turbine installed in cold climate areas, and the second one is the category of the bolts that are unusable as the bolt of the wind turbine installed in cold climate areas; thereby, the criterion of the classification is whether or not the Charpy impact strength as to the material of each bolt is more than or equal to the Charpy impact value 27 [J] (at minus 20° C.), the value 27 [J] as a threshold being derived from the requirement regarding the bolts to be used for the wind turbine installed in cold climate areas.
In order to perform the classification of the bolts, for instance, it may be considered to sample a bolt per each production lot of the chromium molybdenum steel bolts so as to perform Charpy impact test as to the sampled bolt; if the Charpy impact value as to the sampled bolt satisfies the criterion, then the bolts belonging to the production lot are regarded as being usable for the cold climate condition; if not, the bolts belonging to the production lot are regarded as being unusable for the cold climate condition.
In this way, it becomes unnecessary to produce the two kinds of steel materials for the cold climate condition and the non-cold climate condition in separated facilities; thus, the two kinds steel materials for the cold and non-cold climate conditions can be prepared without increasing the initial cost and the running expense.
On the other hand, in a Charpy impact test, a test piece of a right prism shape with a notch is tested whereby the test piece is broken by a high-speed impact; on the basis of the impact energy needed to break the test piece, the toughness of the test piece is evaluated. Accordingly, in order to perform a Charpy impact test, the sampled bolt sampled out of the production lot of the bolts has to be cut so as to manufacture the test piece of the prism shape; thus, complicated and time-consuming work is required in preparing the test piece. Hence, performing the Charpy impact test for the chromium molybdenum steel bolts and classifying the chromium molybdenum steel bolts into two classes (i.e. the usable bolt and the unusable bolt for the cold climate condition) on the basis of the result of the Charpy impact test require the work for hours; and, the complicated work processes become necessary.
Moreover, there is another problem; according to the above-described approach in which whether or not the manufactured chromium molybdenum steel bolts are usable for the cold climate condition by use of the Charpy impact test, the judgment as to whether or not the bolts of the lot are usable cannot be made until the Charpy impact test is finished. Further, since the ratio of the number of the usable bolts to the number of the whole manufactured bolts is not clear before the Charpy impact test is performed, it cannot be judged whether or not a necessary number of the bolts that are usable for the cold climate is prepared; in some cases, a necessary number of the bolts cannot be prepared in time for the construction work of the wind turbine.